The present invention relates to fiber optic transmission systems. More particularly, it relates to an apparatus and method for improving light amplification and noise figure in fiber optic links by using a high-output-power, low relative intensity noise (RIN) optical sources.
Analog photonic links are attractive for remote distribution of optical signals (referred to as xe2x80x9coptical remotingxe2x80x9d) and control of phased array antenna systems due to their low-loss, large intrinsic bandwidth, and immunity to electromagnetic interference. These characteristics of fiber optic links make them desirable for communicating information at RF frequencies including making them as valuable tools for remoting antennas. Photonic links between remote antennas and control stations may be used to remote local oscillator signals or to use RF signals transmitted from an antenna to a remote centralized location. The complexity of the signals received from the remote antennas requires efficiently relaying the information while processing it.
Although analog photonic links may be used as signal processing tools, the complexity of the received RF signals when combined with the dynamic range requirements of antennas imposes strain on the noise figure requirements of photonic links. For example, a typical photonic link composed of a 30 mW DFB laser, a 5 Volt Vxcfx80 MZM, and a photodetector, as illustrated in FIG. 1, may have a RF noise figure of greater than 30 dB with no microwave preamplifier circuitry.
The RF noise figure may deteriorate when laser sources with a high relative intensity noise(RIN) are used in photonic links. Although using high power solid state lasers enable shot-noise-limited photocurrents, and may reduce the noise figure, this approach may be economically prohibitive, thus making high-power shot-noise-limited microwave links uneconomical.
In one approach, Erbium Doped Fiber Amplifiers (EDFAs) are used as in-line amplifiers in fiber transmission systems because of their wide bandwidth and low noise figure. These amplifiers are typically distributed throughout a transmission system in order to maintain high optical power while minimizing signal-to-noise ratio degradation. EDFAs, however, generate amplified spontaneous (ASE) noise, which limits a system""s electrical signal-to-noise ratio through spontaneous-to-spontaneous and signal-spontaneous beat noise at a receiver, which may typically be a photodetector. Because of the wide bandwidth of EDFAs, the ASE may extend well past 1 THz. The beat noise resulting from ASE leads to a large RF noise figure, thus limiting their use in microwave photonic links.
Brillouin amplifiers use stimulated Brillouin scattering to produce large gains over a narrow bandwidth. Brillouin scattering is the inelastic interaction of light with acoustic phonons that result in a shift in frequency of the scattered radiation. This effect is similar to Raman scattering, but information is obtained about acoustic branches of the phonon spectrum. It results from statistical density fluctuations due to acoustical vibrations in the scattering medium. These fluctuations travel at the local speed of sound and the frequency of the scattered light is Doppler-Shifted. The relative velocity of the density fluctuation waves depends on the angle of observation, ranging from the speed of light to zero. Therefore the amount of the frequency shift also varies with the angle of observation.
Stimulated Brillouin Scattering (SBS) is a highly efficient mechanism for producing large gains ( greater than 30 dB) over a very narrow bandwidth ( less than 30 MHz). Considering an electrorestrictive material where acoustic noise is due to the Bownian motion of its molecules (thermal noise), SBS occurs via interaction of an incident light wave (pump beam), an acoustic wave, and the backscattered (Stokes) lightwave. The backscattered light (Stokes light) propagates in a direction opposite to the pump light/beam to interfere with the pump beam. When two lightwaves of different optical frequencies counter-propagate in the same electrorestrictive material, their superposition yields interference fringes, creating zones of high electrical field intensity and zones of weaker field intensity. Due to electrorestriction, periodical compression zones moving at the speed given by the optical frequency difference are induced in the material. If this speed corresponds to the speed of sound in the material, an acoustic wave is created. Thus, due to the electrorestriction, an acoustic wave is generated that stimulates the Brillouin scattering even more, thereby reinforcing the acoustic wave.
The backscattered light wave (Stokes light) is shifted by the acoustic photon frequency of the optical fiber, or the Brillouin frequency. In optical fibers, a significant portion of the optical power traveling through the fiber is converted into the Stokes wave traveling in the opposite direction to the pump beam. Although fiber based Brillouin amplifiers have a narrow gain bandwidth, the baseband noise in the Stokes light may include strong frequency components to the order of 1 GHz, thus impairing their performance in microwave photonic applications where an amplifier is used for amplifying signal information.
Thus, there is a need to overcome the problems encountered by the prior art by enabling the use of low cost high RIN optical sources for microwave photonic links.
Accordingly, the present invention provides an exemplary method and apparatus in order to enable the use of low-cost high RIN optical sources for microwave photonic links by combining the widegain-bandwidth of an EDFA with the narrow gain-bandwidth of a Brillouin amplifier to produce an inexpensive light source for photonic links. The Brillouin gain is preferably used as a carrier amplifier for the present invention.
The exemplary hybrid Brillouin/EDFA (xe2x80x9chybrid amplifierxe2x80x9d) apparatus of the present invention includes at least two couplers, a phase modulator, an Erbium-doped fiber, a laser source to pump the Erbium fiber, an optical circulator, and a length of fiber used for Brillouin amplification. Optical signals from the laser source are split into two optical paths by a polarization maintaining coupler, such as, for example, a 3 dB coupler. Optical signals passing through a first path are amplified by the Erbium doped fiber, and the amplified signals are passed through the optical circulator before passing the optical signals into one end of a spool of optical fiber to pump the Brillouin acoustic wave created in the optical fiber. Optical signals that follow a second path are phase modulated in a phase modulator, such as, for example, a LiNbO3 phase modulator, at the Brillouin frequency and injected into an opposite end of the optical fiber spool to provide input (xe2x80x9cseedxe2x80x9d) to the Brillouin amplifier and improve its low frequency stability. The output of the Brillouin section of the hybrid amplifier is directed through the optical circulator, and the output from the circulator may be used as an optical source for a photonic link.
The present invention also provides a method for improving the noise figure, dynamic range, and loss in fiber optic links. In addition, the present invention enables inexpensive, high RIN sources to be used in analog photonic links.
In one embodiment, the present invention comprises a hybrid amplifier with a DFB laser as a pump with a photodetector attached to the output of the hybrid amplifier.
Still other objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.